U.S. Pat. No. 5,080,986 relates to a magnetic encapsulated dry toner comprising a fluorocarbon-incorporated poly (lauryl methacrylate) as core binder. The capsules shell is either polyurea, polyamide, polyurethane, polyester, or mixtures of thereof, formed by interfacial polymerization at the oil (capsules)/water (carrier medium) interface. The capsules were prepared first in an aqueous medium then spray dried to form the dry toner. The spray drying is an ineffective and costly process, during which capsules, ruptures and/or agglomeration took place. In addition, dry capsules produced by this technique show very low yield.
Brit. Patent No. 1,156,653 relates to a process of encapsulating iron oxide by dispersion polymerization, during which the iron oxide is dispersed in an organic liquid together with a monomer (the monomer is soluble in the organic liquid). Upon polymerization (the polymer is insoluble in the organic liquid) the polymer precipitates from the organic liquid, and is absorbed on the surface of the iron oxide particles.
Brit. Patent No. 950,443 relates to a process to encapsulate iron oxide dispersions in organic solvent with a polymeric skin. The polymeric skin is formed by condensation polymerization.
Encapsulated magnetic particles are also disclosed in U.S. Pat. No. 3,627,682. In accordance with that patent, the encapsulating shell wall material is dispersed in a volatile solvent, which solvent is thereafter removed to form the encapsulated particles. Such an approach is undesirable because it involves the requirement of vaporizing a large amount of volatile solvent, which is both costly as well as potentially injurious to the environment. U.S. Pat. Nos. 4,420,540, 4,511,629, 4,696,859, 4,699,817, 4,713,293, 4,844,991, 5,013,602 and 5,032,428 are directed to magnetic recording media that contain nonencapsulated magnetic particles that are dispersed in a binder and then directly applied as a dispersion to a substrate, the dispersion also using volatile organic solvents.
Offset magnetic printing inks typically use iron oxide pigment as a source for magnetic signals. In order to achieve the required signal strength, the offset inks should contain a high loading of the iron oxide pigment (40-65% based on the total weight of the ink), but such high loading of iron oxide pigment adversely affects the performances of the magnetic printing inks, such as printability, press stability, ink transfer on the roller, ink water balances, drying time, as well as the stability of the magnetic signal.
In addition, the high loading of the iron oxide pigments make it necessary to pass the ink several passes over the three roll mill, at high pressure between the rollers, in order to disperse the magnetic pigment in the ink's vehicle. The three roll mill does not produce an agglomeration-free magnetic inks, due to the lack of chemical affinity between the pigment particles and the ink vehicle. In addition, this mechanical dispersing aid is very costly and time consuming, which adds to the cost of the final ink. Other mechanical dispersing aids, such as a SCHOLD mixer, does not completely eliminate pigment agglomeration. Agglomerations of magnetic iron oxide pigments reduce the magnetic signal strength, and produce an ink with poor runnability on the press. In order to achieve the required signal strength, more inks must be applied during printing. This in turn increase the press problems of the ink and has severe adverse effects on the print quality and the drying time.
Surface treatment techniques known in the prior arts do not produce a uniformly coated or treated magnetic particle. Performances of magnetic ink made with the commercially available treated, or untreated, iron oxide are marginal, lacking long run stability and nonreproducible. The resultant magnetic inks are stiff, short and lack of the proper transfer on the ink roller of the printing presses. In addition, the surface treatment itself is a very costly process.
Magnetic recording system are usually coated from a solvent based dispersion of magnetic particles. These dispersion required enormous amount of energy for curing. In addition undesirable air pollution accompany such the coating processes.
Encapsulated iron oxide and other magnetic particles for liquid toner and nonimpact printer applications contain large amounts of solvent which make them unsuitable for the lithographic printing inks.
From the foregoing it is clear that a significant need exists for encapsulated magnetic particles that may be used in any of the aforementioned application areas, such as printing (lithographic, offset, and the like), magnetic storage (tapes, discs, and the like), and others.
There also exists a significant need for making encapsulated magnetic pigments that can be used in the aforementioned application areas, that does not employ the use of volatile organic solvents. Further, such a method should be able to make encapsulated magnetic particles that have a high percentage of metal in the encapsulated product.
Additionally, there is a need for inks, such as for lithographic printing, that have a high percentage of encapsulated magnetic particles contained therein, that are free of volatile organic solvent, and can be employed without significant agglomeration of the magnetic particles. There is also a similar need for a coating composition for making magnetic recording media, wherein the coating composition has a high percentage of encapsulated magnetic particles contained therein, is free of volatile organic solvent, and can be employed without significant agglomeration of the magnetic particles.
Additionally, there is a need for a method of making inks that can avoid the costly preparation steps that are usually required, such as the flushing process that is employed in the manufacture of ink using flushed pigments or carbon black dispersion.